This invention relates to adjustable waveguide branches, and to directional or hybrid couplers using such branches, and particularly to such branches and couplers using photoconductive materials to affect the division or coupling factor in response to light.
Hybrid or directional couplers are in widespread use for communications systems, providing power division and combining, and also providing differential phase shifts. U.S. Pat. No. 4,588,958, issued May 13, 1986 in the name of Katz et al, for example, describes a predistortion circuit which uses a 4-port, 3 dB, 90.degree. directional coupler. Many other uses exist in the art. It should be noted that one of the four ports of such a hybrid coupler may be terminated in a resistive load, so it may have the appearance of a three-port coupler, even though it is fundamentally a four-port coupler. Such a coupler is described in U.S. Pat. No. 4,906,952 issued Mar. 6, 1991 to Praba et al.
Communications systems have been requiring progressively greater bandwidth in order to handle increasing information throughput. One way to increase the useful bandwidth of a system is to raise the operating frequency, as long as the percentage bandwidth is maintained. Thus, a system operating at a center frequency of 1 GHz with a 10% bandwidth has a useful bandwidth of 100 MHz, while if it could be operated at a center frequency of 10 GHz, the same 10% bandwidth would yield a 1 GHz information-carrying bandwidth. Thus, there is a continuing drive toward use of higher-frequency systems.
Satellite communications systems are by now well known, and use and reliance on such systems continues to grow. In response, the technology has been pushed to raise satellite communications systems from C band (about 5 GHz) towards X and K bands (8 to 12 and 10 to 15 GHz, respectively). Yet higher frequencies may be expected in the future. At these higher frequencies, transmission-line losses tend to be greater than at C-band and below. Also, it is more difficult to generate large amounts of power at high frequencies compared with low frequencies. Satellite communications systems often use hollow "waveguide" transmission lines for X and K-band when runs of significant length are required, even though it may be heavier and more difficult to fabricate and route than coaxial cable (coax). It should be noted that any transmission line may be termed a "waveguide", but hereinafter the term is used to describe hollow transmission lines having a conductive periphery. Waveguide is preferred to some other transmission lines because waveguide can achieve lower transmission loss. On the other hand, for very short runs where great losses are unlikely, as for example within integrated circuits which may be used for signal processing, strip transmission lines (stripline, microstrip) or their equivalent are often used.
In order to maximize the use of the available bandwidth in satellite communications systems, multiplexing schemes are used, by which, for example, polarization and frequency diversity are used in combination to aid in isolating communication channels from each other. The multiplexing schemes make use of hybrid or directional couplers, as described for example in U.S. Pat. No. 5,025,485, issued Jun. 18, 1991 in the name of Csongor et al.
At high frequencies, wavelengths are small, and standard manufacturing tolerances tend to become larger in terms of wavelength than would be the case at lower frequencies. This in turn means that it is more difficult to accurately fabricate a coupler to a specific coupling factor at higher frequencies, and it also means that, in the context of coupler ports which are intended to be mutually isolated, a given level of isolation may be difficult to achieve. U.S. Pat. No. 4,679,011, issued Jul. 7, 1987 in the name of Praba et al, describes a manufacturing technique by which replaceable blocks are used as an aid to achieving the desired coupling factors. This scheme allows the coupling to be set during manufacture by assembling the system with a set of blocks, and by disassembling and changing the blocks if the coupling is incorrect. Another scheme is described in U.S. Pat. No. 4,635,006, issued Jan. 6, 1987 in the name of Praba, in which the walls of a through waveguide of a directional coupler are distorted by pressure in order to affect the coupling factor. This allows the coupler to be adjusted to some degree after manufacture.
However, neither of these schemes allows the coupling to be changed in a simple manner when the coupler is at a remote location, such as a spacecraft in orbit. Such a change of coupling may be desirable to ameliorate the effects of frequency shifts due to damage or age, interference at particular frequencies which might make it desirable to optimize port-to-port isolation at a particular frequency, and other imponderables. Such a change of coupling could also be used to trim an antenna beam-forming network to redirect an antenna beam.